Method of manufacturing non-quenched and tempered steel product

ABSTRACT

This invention relates to a method of manufacturing a non-quenched and tempered steel product, including hot forging a steel material at about 1150˜1250° C. so that the volume thereof is 120% or less, rapidly cooling the hot forged material to about 650˜700° C. at a cooling rate of about 10° C./s or more, and performing warm coining at a temperature of about 600° C. or higher.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. §119(a) priority to Korean Application No. 10-2011-0052563, filed on Jun. 1, 2011, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a non-quenched and tempered (non-QT) steel product, particularly wherein the strength of the non-QT steel product is enhanced, and wherein the manufacturing process is drastically simplified.

2. Description of the Related Art

Enhancing the strength of products that include connecting rods made of a non-QT steel composition not subjected to quenching and tempering typically includes methods such as adding alloying components to the composition, and performing forging followed by controlled cooling to provide a fine-grain structure.

In order to enhance strength by adding alloying components, however, material costs increase significantly and, thus, provides limitations. In the case of forging followed by controlled cooling, the method is limited because the fastest cooling rate that can form a fine-grain structure is 5° C./s or less.

As shown in FIG. 1, a conventional connecting rod is deformed by hot forging, and is then subjected to shot blasting, cold coining so as to correct dimensions of bent portions and large/small ends, stress relief annealing for relieving residual stress produced upon coining, and then shot blasting so as to remove scales and make the surface smooth. However, this method is problematic in terms of increased manufacturing cost due largely to the two shot blasting processes and the stress relief annealing. Furthermore, an insignificant enhancement in strength is achieved.

According to conventional forming processes, forging is performed by introducing a rolling material between upper and lower molds having the shape of the product, and applying pressure thereto using a press, thus obtaining a desired shape. As shown in FIG. 2, a final shape is obtained by carrying out two or three pressing processes, where typically three pressing processes are carried out which include buster, blocker, and finisher processes. When the volume of a final product is 100%, the volume is controlled to 120˜130% upon buster, 110˜120% upon blocker, and 100% upon finisher. For reference, the buster process greatly compresses the material to remove scales and keeps the forging ratio appropriate. After the buster process, the blocker process is performed so as to shape the material to its approximate final shape, thus making it possible to more efficiently perform the finisher process. The volumes at these respective steps are determined via simulation, including mold analysis, etc., upon designing the mold.

The shot blasting process is performed by spraying fine particles of metal or non-metal, called shots or grits, onto the surface of a product to remove and mill scales. As shown in FIG. 3, cold coining is conducted by placing the cooled forged product between coining molds and pressing it under a predetermined amount of pressure. This corrects the bending portions and controls the thickness of large or small ends. This process is used to improve the bending portion of a connecting rod or the like, not to form a novel shape.

This related art is merely utilized to enhance understanding about the background of the present invention, and will not be regarded as conventional techniques known to those having ordinary knowledge in the art.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind the above problems encountered in the related art, with an object of providing an improved method of manufacturing a non-QT steel product. In particular, the method includes rapid cooling to providing a fine-grain structure and warm coining for hardening the structure, thus enhancing the strength or the product. Further, the methods may omit continuous stress relief and shot blasting, thus increasing productivity and reducing the manufacturing cost.

An aspect of the present invention provides a method of manufacturing a non-QT steel product, comprising hot forging a steel material at a heightened temperature (e.g., about 1150˜1250° C.) so that the volume thereof is about 120% or less; rapidly cooling the hot forged material (e.g. cooling from the heightened temperature to about 650˜700° C. at a cooling rate of about 10° C./s or more; and performing warm coining at a suitable temperature (e.g. about 600° C. or higher.

Another aspect of the present invention provides a method of manufacturing a non-QT steel product, comprising hot forging a steel material at a heightened temperature (e.g., about 1150˜1250° C.) so that the volume thereof is about 120% or less; rapidly cooling the hot forged material; and warm coining four faces(top/bottom/left/right) of the material at a suitable temperature (e.g. about 600˜680° C.) so that the volume thereof is about 100˜120%.

A further aspect of the present invention provides a method of manufacturing a non-QT steel product, comprising hot forging a steel material at a heightened temperature (e.g., about 1150˜1250° C.) so that the volume thereof is about 120% or less; rapidly cooling the hot forged material (e.g. cooling from the heightened temperature to about 650˜700° C. at a cooling rate of about 10° C./s or more); and warm coining four faces (top/bottom/left/right) of the material at a suitable temperature (e.g. about 600˜680° C.) so that the volume thereof is about 100˜120%.

In accordance with various embodiments, hot forging may be performed by processing the steel material so that the volume thereof is about 140% or less upon buster, about 130% or less upon blocker, and about 120% or less upon finisher.

In some embodiments, warm coining may further comprise transporting the coined material into a warm holding furnace at a suitable temperature (e.g. about 550˜650° C.) so that residual stress is relieved.

In some embodiments, transporting may further comprise performing shot blasting to remove scales from the transported material.

According to embodiments of the present invention, the material may be a connecting rod for automobiles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional process of manufacturing a connecting rod;

FIG. 2 shows the shape of a material produced using conventional hot forging;

FIG. 3 shows a conventional cold coining process;

FIG. 4 shows a process of manufacturing a connecting rod according to an embodiment of the present invention; and

FIG. 5 shows a warm coining device used for the process of manufacturing the non-QT steel product of FIG. 4.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, embodiments of the present invention regarding a method of manufacturing a non-QT steel product will be described in detail while referring to the accompanying drawings.

It is understood that the term “automobile” or other similar term as used herein is inclusive of motor vehicles in general such as passenger vehicles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g., fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. About can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from context, all numerical values provided herein are modified by the term about.

FIG. 4 shows a process of manufacturing a non-QT steel product according to an embodiment of the present invention. According to embodiments of the present invention, the method of manufacturing a non-QT steel product comprises hot forging a steel material at a heightened temperature (e.g., about 1150˜1250° C.) so that the volume thereof is 120% or less, rapidly cooling the hot forged material (e.g., cooling from the heightened temperature to about 650˜700° C. at a cooling rate of about 10° C./s or more), and performing warm coining at a suitable temperature (e.g., about 600° C. or higher).

In addition, the method of manufacturing a non-QT steel product according to embodiments of the present invention comprises hot forging a steel material at a heightened temperature (e.g. about 1150˜1250° C.) so that the volume thereof is about 120% or less, rapidly cooling the hot forged material, and warm coining the four faces (top/bottom/left/right) of the material at a suitable temperature (e.g., about 600˜680° C.) so that the volume thereof is about 100˜120%.

In addition, the method of manufacturing a non-QT steel product according to embodiments of the present invention comprises hot forging a steel material at a heightened temperature (e.g. about 1150˜1250° C.) so that the volume thereof is about 120% or less, rapidly cooling the hot forged material (e.g. cooling from the heightened temperature to about 650˜700° C. at a cooling rate of about 10° C./s or more), and warm coining four faces (top/bottom/left/right) of the material at a suitable temperature (e.g., about 600˜680° C.) so that the volume thereof is about 100˜120%.

Thus, the method of manufacturing a non-QT steel product according to the present invention generally includes hot forging, rapid cooling, and warm coining. One specific example of the non-QT steel product of the invention is a connecting rod for automobiles. In the case of such a connecting rod, a conventional cold coining process for treating bent portions thereof may be omitted from embodiments of the present invention, thus providing economic benefits.

The non-QT steel composition of the present invention comprises about 0.30˜0.60 wt % of C, about 0.50˜2.0 wt % of Si, about 0.80˜1.40 wt % of Mn, about 0.005˜0.10 wt % of P, about 0.05˜0.10 wt % of S, about 0.050 wt % or less but exceeding 0 wt % of Al, about 0.40 wt % or less but exceeding 0 wt % of Cr, about 0.05˜0.35 wt % of V, about 0.15 wt % or less but exceeding 0 wt % of Ti, about 0.05 wt % or less but exceeding 0 wt % of Nb, about 0.010˜0.080 wt % of Zr, about 0.030 wt % or less but exceeding 0 wt % of N, and a remainder of Fe and impurities.

In accordance with various embodiments, upon cooling after hot forging (so as to provide a volume of 120% or less), high-pressure nitrogen gas is blown thereon and rapid cooling (e.g. at about 10° C./s or more) is carried out until the temperature of the forged material is 680° C., which is just under the A1 transformation temperature. This results in the formation of fine crystal grains (which increases strength) and removal of scales. Thus, the conventional shot blasting process can be eliminated. As shown in the conventional process of FIG. 1, shot blasting is essentially performed because defects may occur while scales are trapped in the material upon cold coining.

Furthermore, according to embodiments of the invention, warm coining is performed at a temperature just under the A1 transformation temperature on the four faces of the top/bottom/left/right of the material so that the volume is 120% or less. This ensures fine dimensions and removes flash generated upon trimming. AS such, conventional milling of the sides can be omitted. FIG. 5 shows such a warm coining device, in which the material is compressed at the four faces of the top/bottom/left/right thereof so that the volume is controlled from 120% to about 100%. This warm coining process, is different from a conventional coining process in which pressing is only performed at the top and bottom of a material. Thus, relatively simple corrections, such as conventional correction of bending portions (FIG. 3), can be performed. In particular, forming and correcting a material can be simultaneously carried out via heating in a heightened temperature atmosphere and processing the four faces.

Also, upon warm coining, strength may be additionally enhanced as a result of work hardening effects due to plastic deformation. After warm coining, the material is held in a heated belt conveyor furnace, e.g. at about 600° C. for about 20 minutes or longer, to relieve residual stress. As such, the material may be simultaneously transported and strengthened. Further, according to the present invention, stress relief annealing (SRA) using conventional cold coining may be omitted, thus reducing the manufacturing cost.

According to conventional methods, cold coining, shot blasting, and SRA are discontinuously performed using separate devices after hot forging and cooling. This undesirably decreases productivity. In contrast, according to embodiments of the present invention, warm holding is continuously maintained after warm coining, and transporting and strengthening are simultaneously performed, thus increasing productivity. Further, the rapid cooling, warm coining for enhancing strength, and continuous warm holding processes according to the present invention make it possible to omit steps and improve the quality of the product.

In particular, according to embodiments of the present invention, hot forging is performed so that the steel material is processed to a volume of about 140% or less upon buster, to about 130% or less upon blocker, and to about 120% or less upon finisher. In contrast, in conventional methods, the volume is controlled not to 100% but to 120% upon finisher, and the remaining 20% is treated by compression upon subsequent warm coining. Thus, the present inventions makes it possible to omit the shot blasting and cold coining steps. Also, according to the present invention, strength may be additionally enhanced by plastic deformation upon warm coining.

Also, according to various embodiments of the present invention, the above warm coining process may further comprise transporting the coined material into a warm holding furnace at a suitable temperature, such as about 550˜650° C., for relieving residual stress. As such, transport and stress relief can be simultaneously carried out. Such transporting may further comprise shot blasting to remove scales from the transported material, and finally trimming the material. According to the present invention, shot blasting may be performed a single time in contrast with conventional methods.

As described hereinbefore, the present invention provides a method of manufacturing a non-QT steel product. According to the present invention, after a forging process, rapid cooling (e.g. at about 10° C./sec or more) is performed to achieving a fine structure, and warm coining for work hardening is then performed thus enhancing strength. Further, continuous stress relief and shot blasting can be omitted, thus increasing the productivity and reducing the manufacturing process.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

1. A method of manufacturing a non-quenched and tempered steel product, comprising: hot forging a steel material at about 1150˜1250° C. so that a volume thereof is about 120% or less; rapidly cooling, by high pressure liquid nitrogen, the hot forged material to about 650˜700° C. at a cooling rate of about 10° C./s or more; and performing warm coining at a temperature of about 600° C. or higher on the rapidly cooled material, wherein the non-quenched and tempered steel product is produced without spot blasting the material.
 2. A method of manufacturing a non-quenched and tempered steel product, comprising: hot forging a steel material at about 1150˜1250° C. so that a volume thereof is about 120% or less; rapidly cooling the hot forged material, by high pressure liquid nitrogen to a temperature of 680° C. or less; and once the hot forged material has been rapidly cooled to 680° C. or less, warm coining four faces of top/bottom/left/right of the material at about 600˜680° C. so that the volume thereof is about 100˜120% on the rapidly cooled material wherein the non-quenched and tempered steel product is produced without spot blasting the material.
 3. A method of manufacturing a non-quenched and tempered steel product, comprising: hot forging a steel material at about 1150˜1250° C. so that a volume thereof is about 120% or less; rapidly cooling, by high pressure liquid nitrogen, the hot forged material to about 650˜700° C. at a cooling rate of about 10° C./s or more; and once the hot forged material has been rapidly cooled to about 650˜700° C., warm coining four faces of top/bottom/left/right of the material at about 600˜680° C. so that the volume thereof is about 100˜120% on the rapidly cooled material, wherein the non-quenched and tempered steel product is produced without spot blasting the material.
 4. The method of claim 1, wherein the hot forging is performed by processing the steel material so that the volume thereof is about 140% or less upon buster, about 130% or less upon blocker, and about 120% or less upon finisher.
 5. The method of claim 2, wherein the hot forging is performed by processing the steel material so that the volume thereof is about 140% or less upon buster, about 130% or less upon blocker, and about 120% or less upon finisher.
 6. The method of claim 3, wherein the hot forging is performed by processing the steel material so that the volume thereof is about 140% or less upon buster, about 130% or less upon blocker, and about 120% or less upon finisher.
 7. The method of claim 1, wherein the warm coining further comprises transporting the coined material into a warm holding furnace at about 550˜650° C. so that residual stress is relieved.
 8. The method of claim 2, wherein the warm coining further comprises transporting the coined material into a warm holding furnace at about 550˜650° C. so that residual stress is relieved.
 9. The method of claim 3, wherein the warm coining further comprises transporting the coined material into a warm holding furnace at about 550˜650° C. so that residual stress is relieved.
 10. The method of claim 7, wherein the transporting further comprises performing shot blasting for removing scales from the transported material.
 11. The method of claim 8, wherein the transporting further comprises performing shot blasting for removing scales from the transported material.
 12. The method of claim 9, wherein the transporting further comprises performing shot blasting for removing scales from the transported material.
 13. The method claim 1, wherein the non-quenched and tempered steel product is a connecting rod for automobiles.
 14. The method claim 2, wherein the non-quenched and tempered steel product is a connecting rod for automobiles.
 15. The method claim 3, wherein the non-quenched and tempered steel product is a connecting rod for automobiles. 